In order to suppress global warming, utilization of natural gas having less amount of CO2 generation while combusting compared with other fossil fuels is considered to increase from now on. Natural gas produced from strata includes acidic materials such as CO2 and H2S. CO2 and H2S have to be removed before liquefaction because CO2 solidifies in the process of liquefaction and causes clogging of pipelines and a heat exchanger and H2S induces corrosion of installations such as pipelines. Other than the natural gas, coal seam gas, gas gasified from heavy oil, and gas gasified from oil coke and coal show the same problem as above.
As a method of removing CO2 and H2S from these gases, a method is disclosed in which feed gas including CO2 and H2S and water solution of N-methyl diethanolamine are contacted under conditions of from 10 to 100 bars and from 40 to 100° C. (For example, see U.S. Pat. No. 4,553,984).
As another method, a method is disclosed in which feed gas including CO2 and H2S is removed by making use of water solution of hindered amine selected from monosubalkylamino subalkanol as rich solution (For example, see Japanese Patent No. 2824387).
Not only in natural gas but also in coal seam gas and gas gasified from heavy oil that can be processed according to the present invention, concentration of CO2 is usually higher than the that of H2S by the order of from 10 to 1000 times. H2S primarily dissolves in advance in rich solution in the regions from a down stage to a middle stage of an absorber and is removed. It has become clear that such phenomenon occurs that the property of the rich solution changes and H2S that has already dissolved therein is released again in association with absorption of CO2 having extremely higher concentration than that of H2S after the above-mentioned dissolution and removal of H2S. For this reason, in order to decrease the property change of the rich solution in association with absorption of CO2 having high concentration, an excess amount of rich solution has to be circulated, causing a problem that an amount of energy to be consumed for regenerating the rich solution is increased. In order to avoid such problem, a method is known in which the pressure of the rich solution that has absorbed CO2 and H2S is lowered and the rich solution is regenerated by releasing the absorbed CO2 and H2S. Unfortunately, this method has a disadvantage that a predetermined performance cannot be obtained at the time of absorption since the concentration of CO2 and H2S remaining in the regenerated rich solution is high.
An object of the present invention is to provide a method and a device for removing acidic materials, such as CO2 and H2S, in a gas purifying process by contacting gas including CO2 and H2S, such as natural gas, coal seam gas, and gas gasified from heavy oil, with rich solution. The method and the device can reduce the regeneration energy for the rich solution and obtain a treated gas having substantially no H2S concentration.